Dynamic balancing of fans and rotors: in situ or in the workshop, the grade to ask for, and the report

Dynamic balancing is the balancing of a rotating part or assembly in motion: weight is added or removed so that the rotor's mass centreline moves towards its axis of rotation and the rotor vibrates less at running speed.^[13][12]

Unbalance means that the mass centreline and the geometric centreline of the rotor do not coincide. NASA's maintenance guide names the parasitic load from vibration caused by unbalance and misalignment as one of the leading causes of premature rolling-element bearing failure.^[12]

On fans, unbalance often comes from the airstream: the US Department of Energy's fan sourcebook says contaminant build-up on fan blades "is often not uniform, resulting in imbalance problems", and that erosion and corrosion in harsh service can thin the blades and unbalance the impeller, in which case the impeller or the whole fan should be replaced.^[13]

Vibration analysis can detect dynamic imbalance, a bearing problem or a motor winding problem before it gets worse, the same sourcebook notes. For how to buy a survey and read its report, see vibration analysis; for the motor's electrical side, ask an industrial electrician.^[13]

A rotor needs balancing when it is new or rebuilt, after wear or build-up has changed its mass, and when a vibration reading points to unbalance. NASA's guide recommends balance certification of all new and rebuilt machinery, with components such as the rotor, shafts and sheaves balanced individually and then trim balanced as an assembly.^[12][13]

For fans in dusty or wet airstreams, cleaning comes first: the DOE sourcebook says such fans should be cleaned regularly, and notes that cleaning is often done only once dust build-up shows as vibration or poor performance.^[13]

Low vibration alone does not prove a good balance. NASA's guide says "it is also not sufficient to balance a rotor until it achieves low vibration levels", and that precision balance methods "should include the calculation of residual imbalance".^[12]

Balance in situ when the rotor can run in its own bearings and taking it out would cost teardown, reassembly and realignment; use a workshop balancing machine when the rotor is out anyway, is new or repaired, or is a component to be balanced before assembly.^[12][14][4]

Sources:^{[12][14][4][1][3][6][7]}

Ask for the balance quality grade that fits the rotor, under ISO 21940-11:2016: G6.3 for fans, as a 2025/26 South African ventilation specification asks, and for other rotors the grade the table below gives their type.^[1][15][12]

ISO 21940-11:2016 is the ISO standard for rotors with rigid behaviour. It specifies the magnitude of the permissible residual unbalance, the necessary number of correction planes, how the permissible residual unbalance is allocated to the tolerance planes, and how to account for errors in the balancing process.^[1]

ISO 21940-11:2016 replaced ISO 1940-1:2003, which ISO withdrew on 9 November 2016, and was amended in 2022; its latest systematic review closed on 3 September 2026. Rotors with flexible behaviour fall under ISO 21940-12:2016.^[1][2][3]

ISO 1940-1:2003 gave its recommendations for rigid rotors "according to their machinery type and maximum service speed", and specified how to verify the residual unbalance.^[2]

That South African specification is the Mine Health and Safety Council's bid MHSC 011/2025-26 for an air-conditioning and ventilation installation. It requires axial flow fans to be balanced to ISO 1940, citing its 1973 edition, "within grade G6.3", and centrifugal fan impellers and shafts to grade G6.3.^[15]

For fans of less than 300 kW, or driven by a standard electric motor of up to 355 kW, ISO 14694:2003 (amended in 2010) sets balance and vibration limits; its abstract refers more powerful fans to ISO 10816-3, which ISO 20816-3:2022 replaced before its own 2026 revision.^[8][9]

The grade number is a velocity in mm/s: the product of the permissible specific unbalance and the rotor's angular velocity at its maximum service speed. A lower number is a tighter balance, and the permissible residual unbalance grows with the rotor's mass. These examples are from the ISO 1940-1:1986 table, as NASA's 2008 guide reproduces it.^[12]

Sources:^[12]

In practice, a balancing report should show the residual unbalance left in each correction plane against the permissible value for the grade you ordered, so you can check the result yourself.^[12][1]

NASA's guide has the balance quality stated in the quotation, measured by a person trained and experienced in machinery balancing, whose qualifications go with the certification data, and signed by the purchaser's representative.^[12]

ISO 21940-14:2026 sets out how to identify and assess errors in the unbalance measurement and take them into account when evaluating the residual unbalance, and ISO 21940-13:2012 sets reporting and record-keeping requirements for in-situ balancing.^[5][4]

A report you can check holds these items:^[12][1]

• the rotor: tag, type, mass in kg and maximum service speed in r/min, since the permissible residual unbalance depends on both;^[12]
• the standard and grade ordered (ISO 21940-11:2016, grade G6.3, for example) and the permissible residual unbalance for each correction plane;^[1]
• the residual unbalance before and after balancing in each plane, not only a vibration reading;^[12]
• the correction weights and how they are fixed: welded, riveted, or bolted with a hardened bolt and a locking device; NASA's guide accepts clip-on weights only on centrifugal fans, and no stick-on lead weights or solder;^[12]
• for a keyed shaft, the key convention used: half-keys under ISO 21940-32:2012, with the parts marked;^[7][12]
• for in-situ work, the vibration at once running speed (1X) before and after, the narrowband value NASA's default acceptance limits use, since "an overall value is not acceptable";^[12]
• the name and qualifications of the person who balanced the rotor.^[12]

In South Africa, the Driven Machinery Regulations, 2015 make the user cause every shaft, pulley, wheel, gear, coupling and similar revolving part to be "securely fenced or guarded", and every driving belt, rope or chain to be guarded, unless it is in such a position or of such construction that it is as safe as it would be if it were (regulation 3).^[10]

The regulations apply to the operation, repair, maintenance, inspection and testing of driven machinery (regulation 2).^[10]

A balancing company working on your site is a mandatary, which the Occupational Health and Safety Act defines to include "a contractor or a subcontractor for work" (section 1). Section 37(1) presumes that an employer or user did what its employee did, and section 37(2) applies this to a mandatary "except if the parties have agreed in writing to the arrangements and procedures between them to ensure compliance by the mandatary with the provisions of this Act", so put those arrangements in writing with the balancing company.^[11]